The present invention relates to concrete pipe reinforcement. Typically, a circular or eliptical cage is formed of a network of interconnected longitudinal and circumferential wires. Concrete is then cast around this reinforcing cage to form a reinforced pipe.
In larger round or eliptical pipe, it is conventional to provide stirruping in the crown and invert areas of the pipe. Stirruping comprises a plurality of wire projections which project generally radially outwardly from the reinforcing cage. The stirrup projections extend along the length of the cage and extend to either side of the crown and invert from 45 to 60 degrees.
Stirruping used to be achieved by welding separate individual rods to the reinforcing cage at various points along the length and along a lateral or circumferential arc thereof. Others have used sinusoidal shaped stirrup members which extend the length of the pipe and comprise a plurality of projections from a single member. Other comparable stirrup members have also been conceived, and stirrup mats have become popular recently. In a stirrup mat, a network of interconnected wires defines a mat from which a plurality of stirrup projections project. The mat is then shaped to conform to the configuration of a cage in the crown and invert areas and is secured to the cage in those areas. It is designed to encompass 45 to 60 degrees to either side of the crown or invert.
The object of such stirruping is to reinforce the concrete pipe at its greatest tangential tensile stress, namely the crown and invert. As one proceeds to either side of the crown or invert, the tangential tensile stress forces begin to diminish and that is why stirruping is typically required only in the general area of the crown and invert, subtending a 45 to 60 degree angle to either side thereof. While the provision of such stirruping is costly, it is necessary in order to reinforce the concrete pipe sufficiently to prevent cracking of the pipe under load.
Stirruping also presents a problem in that the elliptical cage passes closer to the inside wall of the pipe at the crown and invert, but closer to the outer wall at the spring lines. The stirrups must be sufficiently tall to intercept the line of stress through the bulk of the wall of the pipe. Yet, the tall stirrups at the lateral edges of the 90.degree. to 120.degree. stirruped area tend to project through the pipe wall when the pipe is cast.